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The use of FTIR instruments
The Environment Agency has issued Technical Guidance Note (TGN) M22, within its series of monitoring TGNs, to provide guidance on necessary requirements when using manual extractive Fourier transform infrared (FTIR) spectroscopic instruments to measure emissions of pollutants from industrial stacks. It is not applicable to FTIR used for continuous emissions monitoring.
FTIR is an attractive technique for emissions monitoring due to a number of advantages:
• it can monitor a very broad range of analytes including water vapour • it can sample hot, wet stack gas removing the risk reactive sample losses in drier units (e.g. SO2) • it provides real-time data enabling issues with CEM’s to be identified early during QAL2 / AST testing (EN 14181) rather than several weeks later as with manual methods • it is possible to expand the capability of instruments to monitor additional analytes (e.g. N2O) • it is straightforward to create supplementary concentration ranges • some models are now available with embedded O2 sensors enabling automatic correction to reference conditions
These capabilities make FTIR both powerful and flexible, however the trade-off is that its operation can be more complex than other techniques that are limited to fewer analytes and / or fixed ranges. Consequently, the motivation behind TGN M22 has been to provide guidance to users so that the quality of monitoring possible with FTIR is assured when used as an alternative to several European standard reference methods.
Introduction – scope
TGN M22 provides a method for the use of FTIR to carry out measurements in support of the Waste Incineration Directive (WID) and Large Combustion Plant Directive (LCPD). It stipulates that the requirements described in these Directives and from other relevant European standards are met. It is also applicable to other applications, as it provides instrument requirements and operational procedures to ensure the production of valid monitoring results. It applies to FTIR instruments configured for extractive sampling and measurement.
The requirements for the use of manual extractive Fourier transform infrared (FTIR) spectroscopy for the speciation and quantification of gases monitored within stacks include:
• sampling configuration • annual analyser checks • ongoing quality assurance and quality control Note: TGN M22 is not applicable to continuous emissions monitoring systems.
Principle of operation
The FTIR spectrometer provides a measurement in the infrared (IR) region over a wide spectral band. Analysis of the recorded spectra enables the concentration of a wide number of determinands to be determined simultaneously. In principle any compound that has an absorption spectrum in the IR region can be measured by FTIR. Determinands that cannot be measured by FTIR are symmetrical diatomic molecules, where there is no dipole change upon vibration, such as oxygen and chlorine.
One issue with the analysis of FTIR spectra is that the spectra of different determinands may overlap, leading to potential problems with cross-interference and mis-identification. Water vapour is a common interferent, as it has absorption features across most of the IR region. The user may calibrate the range for a given determinand over as wide a range as desired by acquiring reference spectra at appropriate concentration increments. If the instrument were used at a plant covered by WID, the certification range must be less than 1.5x the emission limit value (ELV) of each pollutant being measured (for other applications it must be less than 2.5x the ELV).
Consequently, it is strongly recommended that testing carried out to demonstrate meeting of the performance requirements across the operational range is designed so that it is also possible to demonstrate meeting of the performance requirements across all likely certification ranges.
Sampling system – general requirements
Sampling shall comply with the requirements of BS EN 15259. A representative volume of gas is extracted from the stack at a controlled flow rate into a sampling system consisting of: • sampling probe • filter • sampling line • temperature and pressure sensors (unless already part of the FTIR analyser) • pump If a hot/wet system is used, the probe, filter box and sampling line will generally be heated to a similar temperature as the gas cell.
Furthermore, the selected temperature shall be at least 20º C above the dew point of the sampled stack gas. All parts of the sampling system positioned upstream of the FTIR analyser shall be made of materials that do not react with or absorb any of the determinands.
Apparatus requirements
Information on materials suitable for sampling can be found under BS ISO 10396. • Sampling probe – this shall be able to convey a representative sample of the stack gas to the FTIR measurement cell. It may contain a filter to remove particulate matter. The probe shall be able to resist absorbance of determinands and be of sufficient length • Filter – a filter of inert material shall be positioned after or as part of the sample probe and be of an appropriate pore size (3mm is usually sufficient) to prevent particulate matter entering the remainder of the system.
The filter shall be periodically changed or cleaned in order to ensure adequate functionality. If the filter is located outside the stack, it shall be heated to at least 20° C above the stack gas dew point temperature • Sampling line – to facilitate system response time the sampling line shall be as short as is practical. The line shall be made of inert material that is able to resist the absorbance of determinands
Characteristics of the analyser and sampling system
The methods given below are SRMs (Standard Reference Methods) for determinands that can be measured using FTIR. If the instrument has been certified under the MCERTS scheme then it is considered suitable for use within this reference method for any determinand included in the certification, provided the instrument has been certified at an appropriate range for the application.
For compliance monitoring, if the analyser does not have MCERTS certification for a determinand covered by an SRM, then equivalency to the appropriate SRM shall be demonstrated by an accredited test house in accordance with BS EN/TS 14793. Furthermore, the test house shall demonstrate that performance criteria under certification/commissioning are met. Detailed information about this is available on the Environment Agency’s website. www.mcerts.net.
Monitoring determinands not covered by a SRM Performance characteristic tests for determinands not covered by a SRM may be carried out either by an approved test house or by the user employing a suitably certified gas delivery system (a liquid injection system may be used for determinands, such as water vapour). For these determinands the minimum performance tests required under this TGN are: • response time • detection limit • lack of fit • interferents
Performance requirements of the method
The uncertainty of the measured value for the method is influenced by the performance characteristics of the analyser and by the: • sampling line • site specific conditions • calibration gases used
Establishment of the uncertainty budget
An uncertainty budget shall be established following Annex E of TGN M22 for each determinand measured. The determined expanded uncertainty shall meet the required maximum allowable overall uncertainty where it exists for a given determinand in accordance with DD CEN/TS ISO 14956 and BS ENV 13005. For substances covered by the WID or LCPD the expanded uncertainty calculated by combining values of standard uncertainties associated with the performance characteristics shall be less than the value specified in the relevant CEN SRMs expressed at the daily emission limit value, on dry basis and before correction to the O2 reference concentration.
Field operation
• Sampling location – the sampling locations shall meet the requirements of BS EN 15259 and provide representative samples. In addition, the sampling location shall be chosen with regard to safety of the personnel, accessibility and availability of electrical power. Further guidance is provided in the Environment Agency’s document TGN M1
• Sampling point – BS EN 15259 describes the approach for selecting a sample point(s)
• Choice of the measuring system – The following characteristics of stack gas shall be known before the monitoring campaign: temperature; moisture content; particulate matter loading; the expected concentration ranges and emission limit values (where applicable); the expected concentration of potentially interfering substances, including at least the components listed in BS EN 15267-3
The measuring system shall have performance data, such as MCERTS certification, to demonstrate that it’s suitable for each determinand in the measurement campaign. To avoid long response times, the sample line should be as short as possible. If necessary a bypass pump may be used. A heated filter appropriate to the dust loading shall be used. The FTIR sample cell shall be kept at the manufacturer’s specified temperature and pressure. Alternatively, pressure and temperature correction is permitted if it’s demonstrated that the relevant performance requirements can be met while this correction is applied. The sampling probe, filter, connection tube and analyser shall be stabilised at the required temperature.
After pre-heating, the flow passing through the sampling system and the analyser shall be adjusted to the chosen flow rate to be used during measurement. Any data recording, data processing and telemetry system used in conjunction with the measuring system shall be checked for proper functioning. If any components are changed, then these checks shall be repeated. All checks shall be documented.
Selection of check gas
Suitable check gases shall be selected (from the determinands, where possible) to demonstrate that the FTIR is capable of producing valid and accurate spectra at the frequencies and absorbance ranges that the analytical model will be using. If check gas cylinders for the determinands are not commonly available, alternative check gases that have equivalent spectral characteristics may be used. Where this approach is used the test laboratories shall justify it in the site specific protocol and monitoring report.
Check gases used for each stack matrix shall meet the following criteria: • Any component specific peak greater than ten times the LOD associated with a check gas, shall lie within ± 25% of the centre of a target analytes analytical wave number band. Where multiple analysis bands are used for a target analyte, this requirement shall be met for at least one of these bands • The absorption bands above shall exhibit peak absorbance greater than ten times the lower detection limit but less than 1.5 absorbance units • At least one absorption validation gas band within the operating range of the FTIR instrument shall have an peak width no greater than the narrowest analyte absorption band.
Perform and document measurements or cite studies to determine analyte and check gas compound line widths In addition, check gases must also be selected to demonstrate that the sample system is free from leaks and can transport the sample to the FTIR without any losses or contamination. The most reactive of all the determinands shall be selected for this purpose and passed through the entire sample system. If check gas cylinders for the most reactive determinands are not commonly available, however, an alternative gas that has equivalent or greater reactivity may be used.
Where this approach is used the test laboratories shall justify it in the site specific protocol and monitoring report. The gas standards, wherever possible, shall all be traceable and certified with a measurement uncertainty of ≤ ± 2% of value with a 95% confidence level. Test laboratories shall have documented procedures, which describe how the above requirements are applied, stating the gases used, the frequencies of the specific peaks, and how these peaks apply to the gases measured.
Start of campaign zero and check gas tests
The zero gas shall be a gas containing no significant amount of the determinands; for example, purified nitrogen or synthetic air (synthetic air being N2 and O2). The zero gas shall be measured once. It is not a requirement to measure the zero gas between each check gas. The following zero and check gas procedure shall be used: • supply zero and check gases through the sampling system at the sampling probe, as close as possible to the nozzle (in front of the filter if possible) • when applying the zero gas, record ten spectra and determine the detection limit for each determinand for the monitoring campaign • when applying the zero and check gases, determine the T90 response time • wait at least three times the T90 value before taking the readings for zero and check gas • the performance requirements shall be met for the zero and check gases
If the zero test does not meet the criteria the zero gas shall be disconnected from the front of the sample probe and connected directly to the inlet of the FTIR analyser gas cell. If the reported concentrations remain unaltered the sampling system is considered free from contamination. The zero gas shall be disconnected from the analyser inlet and reconnected to the front end of the sample probe. Alternatively, if connecting the zero gas directly to the cell inlet results in reported concentrations that decrease to, or better than, the performance requirement, then contamination of the sampling system is indicated. The contamination shall be removed and the zero check test repeated.
Checking the sampling system with the most reactive gas
The sampling system shall be checked with the most reactive target gas being measured. If check gas cylinders for the most reactive determinands are not commonly available, however, an alternative gas that has equivalent or greater reactivity may be used. Where this approach is used the test laboratories shall justify it in the site specific protocol and monitoring report.
The sampling system shall be checked using one of the following techniques: • Dry cylinder gas injection – the most reactive gas is applied directly to the inlet of the analyser at the start of the monitoring campaign and the final stable result recorded. The gas is then applied to the entire sampling system and the final result obtained must be ≤ ± 5% of the value obtained during the direct injection • Wet gas injection – the most reactive gas is applied to the entire sampling system using a wet gas injection system. The final result obtained must be ≤ ± 5% of the value the gas injected into the system • Analyte spiking – spike the most reactive gas into the sampling system upstream of the particulate filter. The recovery of the spiked gas shall be within 30% Emissions monitoring
The probe is placed at the representative point(s) in the stack in accordance with BS EN 15259. The zero gas test shall be carried out at least once per monitoring day. If the monitoring campaign is spread over many days it is recommended that the check gases are measured at frequent intervals in addition to the zero gas. This reduces the risk of drift remaining undetected and exceeding the 5% limit, resulting in data being rejected.
End of campaign zero and check gas tests
Zero and check gas procedure: • remove the probe from the stack • supply zero gas through the probe and wait at least three times the T90 value prior to taking a reading • the zero check performance requirement shall be met • supply the required check gases through the probe and wait at least three times the T90 value prior to taking a reading • the check gas performance requirement shall be met for the required check gases
Determining drift across the monitoring campaign
It is important that all the gases used to check the analyser before and after measurements shall pass specific drift criteria, these are: • < 2% = pass • ≥ 2% & < 5% = apply drift correction to data • ≥ 5% = fail If the drift test is failed the monitoring data shall be rejected.
Verification of water vapour measurement
Water vapour is usually measured continuously with other determinands. It is often required for correcting monitoring results to reference conditions. It also affects other measurements because it has a very broad spectral range. In this TGN, water vapour is treated in the same way as other determinands. Monitoring organisations can obtain accreditation to use FTIR as an alternative method to BS EN 14790 – determination of the water vapour in ducts.
If the monitoring organisation does not meet the requirements of accreditation for water vapour measurements, they shall use BS EN 14790 to measure the water vapour for correction of monitoring results to reference conditions. This measurement shall be done for each stack at least once per day.
Status of this document
This TGN may be subject to review and amendment following publication. The latest version is available at: www.mcerts.net. In addition, there are numerous references, annexes and extra notes, including a post monitoring campaign outlining QA/QC checks.
Implementation date
It is expected that organisations which hold MCERTS accreditation for TGN M22, will have met the requirements of this version of TGN M22 by January 1, 2012. You can read the full details of the TGN M22 document here: http://publications.environment-agency.gov.uk/PDF/GEHO0311BTPY-E-E.pdf.
A new version of TGN M22 is due to be published in the next few weeks.
About the Environment Agency
The Environment Agency is a United Kingdom regulatory authority, whose remit covers England and Wales. Our role is to protect and improve the environment and to promote sustainable development. Protection of the environment relates to threats such as flooding and pollution. Our Operator Self Monitoring Team, which is part of National Operations is responsible for ensuring that the monitoring of emissions to land, air and water meets the requirements of strict European and UK laws. The scope of our work includes setting standards and providing guidance for stack emissions monitoring, ambient air monitoring, chemical testing of soil, monitoring water discharges, monitoring of effluent flow, noise and odour. To give us confidence in the monitoring of emissions to air, land and water by industrial operators and other businesses, we developed MCERTS, our Monitoring Certification Scheme. MCERTS provides for the product certification of monitoring systems (instruments, meters, analysers and equipment), the competency certification of personnel and the accreditation of laboratories under the requirements of European and international standards.
About the National Physical Laboratory
The National Physical Laboratory (NPL) is the UK’s National Measurement Institute and is a world-leading centre of excellence in developing and applying the most accurate measurement standards, science and technology available. NPL has developed standards which underpin an infrastructure of traceability throughout the UK and the world that ensures accuracy and consistency of measurement. Visit: http://www.npl.co.uk/
Author
Rupert Standring Technical Advisor Emissions Monitoring Team (part of National Operations) Environment Agency
Dr Marc Coleman Senior Research Scientist National Physical Laboratory Hampton Road Teddington, Middlesex, UK TW11 0LW
W: www.npl.co.uk/environmental-measurement
Published: 01st Mar 2012 in AWE International
